I really like the simplistic visuals you went with! I think having the planets and star just be circles gives the game its own unique charm and aesthetic without being super distracting! I also think that your core mechanic of stacking orbits has a lot of potential that extends past just a survive as long as you can game. I could see this being a really fun level based puzzle game, new levels could introduce more complex mechanics like binary star systems, and stars/planets with differing masses and thus differing gravitational pulls. I like the games SFX, but I do wish that there was at least some background music, nothing complex, just something that would help further set the mood as being in space(I'm thinking something like the music in Universe Sandbox)
Pretty good game though nonetheless!